Feature Request: Support provisioning of code snippets in generated Xcode projects

Some companies/projects provide shared code snippets checked in to source control. Right now, we perform a separate step to copy the code snippets to the appropriate location.

  cp -R \
    "${SRC_ROOT}/CodeSnippets/." \
    "${HOME}/Library/Developer/Xcode/UserData/CodeSnippets"

Granted this request is slightly orthogonal to the generation of the project. However, the generation of the project does provide a nice signal to copy the shared code snippets.

Options

1. Do nothing and suggest clients wrap calls to the project generation in a shell script which does whatever is necessary to provision the development environment.
2. Do nothing and suggest clients use a multirun rule to execute the project generation and run a target that copies the code snippets.
3. Add an attribute listing the code snippets that should be copied to the user's Library directory.
4. Provide attributes that allow a client to specify pre- and post- install hooks. The installer would then run them in the specified order before and after the installation. (NOTE: This is really just incorporating option 2 into rules_xcodeproj.)

The issue names no files, tests, or entry points; start by locating the generated Xcode project installation flow and its existing tests. Clarify which of the four provisioning options is wanted, then define the supported behavior and verify it with coverage for code-snippet installation.